1. Field of the Invention
The invention relates to a reflection type liquid crystal display apparatus, which apparatus having: a liquid crystal device of a reflection type for reflecting incident light by changing the state of polarization in accordance with an image; and a polarizing prism for emitting the light of a predetermined polarized component in the light irradiated from a light source and inputting the light into the reflecting type liquid crystal device and for emitting the light of the predetermined polarized component in the reflected light from the reflecting type liquid crystal device, thereby obtaining a projection image.
2. Description of Background Information
FIG. 1 shows an image display apparatus which uses a liquid crystal light valve of the photoconductive type (hereinafter, simply referred to as a light valve) disclosed in Japanese Patent Application Kokai Nos. H2-22627, H2-69721, H4-163427, filed by the same applicant as the present invention, as a reflecting type liquid crystal panel. The apparatus receives a read-out light from a light source and changes the state of polarization of the read-out light in accordance with the image formed on a photoconductive layer, thereby obtaining a projection image.
As shown in the figure, such a display apparatus is configured that a CRT (cathode ray tube) 1R for a red color to generate a red image and a photoconductive type liquid crystal light valve (hereinafter, simply referred to as a light valve) 2R for the red color which uses the red image as writing light are coupled to sandwich an optical fiber 3R, thereby forming a first color channel. In the similar manner, a second color channel is formed by a CRT 1G for a green color, a light valve 2G for the green color, and an optical fiber 3G. A third color channel is formed by a CRT 1B for a blue color, a light valve 2B for the blue color, and an optical fiber 3B.
On the other hand, the incident light to each of the light valves in the first to third color channels described above is irradiated from a light source 4. The incident light is first led to a polarizing prism 5. Only the s-polarized (Senkrecht polarized light) component in the incident light is bent by the polarizing prism 5 in the direction perpendicular to the progressing direction and leads to a dichroic mirror 9.
The dichroic mirror 9 has special characteristics such that the green light is reflected and the red and blue light is transmitted. The red and blue light which is transmitted through the dichroic mirror 9 reaches a dichroic mirror 10. The reflected green light is further reflected by a total reflecting mirror 11 and is led to the light valve 2G for the green color. The dichroic mirror 10 has spectral characteristics such that the blue light is reflected and the red light is transmitted. The dichroic mirror 10 leads the light of those primary colors to the light valve 2B for the blue color and to the light valve 2R for the red color, respectively.
Each light valve reflects the read-out light incident as an s-polarized light, including a p-polarized (polarized light) component according to a CRT output image, thereby executing a light modulation. The reflected read-out light, namely, the output projection light of each color channel again enters the polarizing prism 5 through the dichroic mirrors 9 and 10 and is synthesized to one image. The polarizing prism 5 transmits only the p-polarized light according to the image in the synthesized projection light and projects the image onto a screen through a projection lens (not shown).
In the display apparatus, for example, a metal halide lamp is used as a light source 4. This lamp has an arc length of about 5 mm. The outgoing light obtained by reflecting the emitted light by a parabolic mirror will become slightly inclined with respect to an optical axis LO as shown in FIG. 2 and the inclined light will enter the polarizing prism 5.
FIG. 3 shows polarizing characteristics of the prism 5. In the diagram, an axis of abscissa .lambda. [nanometers] denotes a wavelength of the light which enters the prism 5. An axis of ordinate T [%] indicates a transmittance of the incident light. In the diagram, T.sub.p denotes transmittance characteristics of the p-polarized component in the incident light and T.sub.s indicates transmittance characteristics of the s-polarized component in the incident light. From this characteristics diagram, it will be also understood that the polarizing prism 5 has the polarizing characteristics such that the p-polarized component is transmitted and the s-polarized component is reflected.
There is a phenomenon such that, when a slightly inclined light beam enters such a polarizing prism from the metal halide lamp as mentioned above, the plane of polarization of the light beam other than the light beam of a special azimuth (direction that is parallel to the plane of incidence of the incident light beam that is parallel to the optical axis) rotates as will be explained hereinbelow. There is, consequently, a problem such that the contrast of the image is deteriorated.
More specifically, as shown in FIG. 4, if we assume that the light source 4 is a so called point light source P, a light beam PQ that is parallel to the optical axis LO enters the polarizing prism 5. At a boundary surface (hereinafter, referred to as surface of polarization operation) 5a of the polarizing prism 5, the prism 5 transmits the p-polarized light in the incident light beam PQ to obtain the light beam L.sub.p and reflects the s-polarized light to obtain the light beam L.sub.s.
The polarizing state of the transmitted light beam L.sub.p at the surface of polarization operation 5a in this instance is diagrammatically shown in the part A in FIG. 5. A polarizing state of the reflected light beam L.sub.s is diagrammatically shown in the part A in FIG. 6. In FIGS. 4 to 6, each polarizing state is shown by using common coordinate axes X, Y, and Z. The plane including the coordinate axes X and Z is parallel to a so called plane of incidence including a normal line QR at an incident point Q on the surface of polarization operation 5a, the incident light beam PQ, and the reflected light beam L.sub.8. The plane including the coordinate axes Y and Z crosses the plane of incidence at right angles and is parallel to the optical axis LO. As shown in the part A in FIG. 5, the transmitted light beam L.sub.p is a linear polarization light whose direction of polarization is included in the plane of incidence and is perpendicular to the optical axis LO, so that it will be understood that the plane of polarization of the transmitted light beam L.sub.p is in the direction that is parallel to the coordinate axis X. As shown in the part A in FIG. 6, since the reflected light beam L.sub.s is a linear polarization light whose direction of polarization is perpendicular to the plane of incidence and is perpendicular to the optical axis LO, it will be understood that the plane of polarization of the reflected light beam L.sub.s is in the direction that is parallel to the coordinate axis Y.
On the other hand, the prism 5 also receives light beams shown by arrows B, C, D, and E in FIG. 4 each is inclined with respect to the optical axis LO. At the surface of polarization operation 5a, the prism 5 transmits the p-polarized light in the incident light beams B, C, D, and E and reflects the s-polarized light in a manner similar to the incident light beam PQ that is to the optical axis LO. The state of polarization of the transmitted light beam at the surface of polarization operation 5a when the light beam B is incident is schematically shown in the part B in FIG. 5 and the state of polarization of the reflected light beam is shown in the part B in FIG. 6. The state of polarization of the transmitted light beam at the surface of polarization operation 5a when the light beam C is incident is schematically shown in the part C in FIG. 5 and the state of polarization of the reflected light is shown in the part C in FIG. 6. The state of polarization of the transmitted light beam at the surface of polarization operation 5a when the light beam D is incident is schematically shown in the part D in FIG. 5 and the state of polarization of the reflected light is shown in the part D in FIG. 6. The state of polarization of the transmitted light beam at the surface of polarization operation 5a when the light beam E is incident is schematically shown in the part E in FIG. 5 and the state of polarization of the reflected light beam is shown in the part E in FIG. 6.
As shown in the parts E and C in FIG. 5, the transmitted light beam is the linear polarization light included in the plane of incidence and each incident light beam is not parallel to the optical axis LO. The plane of incidence, however, which is formed to include the normal line standing at the incident point on the surface of polarization operation 5a and the incident light beam E or C, is parallel to the plane of incidence of the incident light beam PQ that is parallel to the optical axis LO as mentioned above. It will, accordingly, be understood that the plane of polarization of the transmitted light beam is also in the direction that is parallel to the coordinate axis X in a manner similar to that shown in the part A in FIG. 5. As shown in the parts E and C in FIG. 6, since the reflected light beam is a linear polarization light perpendicular to such a plane of incidence, it will be also understood that the plane of polarization of the reflected light is also in the direction that is parallel to the coordinate axis Y in a manner similar to that shown in the part A in FIG. 6.
As shown in parts B and D in FIG. 5, on the other hand, the transmitted light beam is a linear polarization light included in a plane of incidence and each incident light beam is inclined with respect to the optical axis LO in the direction of Y-axis (also including the negative direction: the same shall also be applied to the description given hereinbelow). The plane of incidence which is formed to include the normal line at the incident point on the surface of polarization operation 5a and the incident light beam B or D is inclined with respect to the plane of incidence which is formed by the incident light beam PQ that is parallel to the optical axis LO mentioned above in the direction of X-axis. It will, accordingly, be understood that the plane of polarization of the transmitted light beam is also in the direction that is inclined with respect to the coordinate axis X in accordance with such an inclination. As shown in the parts B and D in FIG. 6, since the reflected light beam is a linear polarization light perpendicular to the plane of incidence having an angle for the coordinate axis X, it will be also understood that the plane of polarization of the reflected light is in the direction that is inclined for the coordinate axis Y in accordance with such an angle.
As mentioned above, the plane of polarization is rotated in each of the light beams other than the light beams (light beams C, E, and the like) which are incident in parallel to the plane of incidence of the incident light beam PQ that is parallel to the optical axis LO. In spite of the fact that the plane of polarization should inherently be rotated in accordance with only the image formed on the light valve, therefore, the plane of polarization is unnecessarily rotated by the polarizing prism 5 which is not directly related to the formation of the image and such a rotation will cause the deterioration of the contrast of displayed images.